Audio pitch control



June 28, 1960 J. c. R. LlcKLlDER Aunro PrrcH coN'rRor.

2 Sheets-Sheet 1 Filed NOV. 7, 1957 Y June 2 8, 1960 .1.c. RQ LlcKLlDER 2,943,152

AUDIO PITCH CONTROL Filed Nov. 7, 1957 2 sheees-sneet 2 INVENTOR.

2 Jasfpx/ cena/a :e

BY. what* United States llatent 8 Claims. (Cl. 179-155) This invention relates to control of the pitch of audio signals, and particularly to a method and system whereby the fundamental frequency of an audio signal such as speech or-music is automatically altered, to achieve a desired subjective pitch pattern.

The present invention may be utilized in a communication-system `in such manner that a measure of secrecy relating to the identity of the speaker be obtained. For example, by automatically lowering the pitch of a voice,

` it is possible to prevent arecognition ofa voiceas that of male or female. In addition thereto,` a` particular individuals pitch frequency may be altered sorthat his identity would be concealed.

`In accordance with the present invention, a method and system is provided whereby the subjective pitch of a voice or other quasi-periodic signal is automatically lowered.

The aforementioned method and system produces a lowering of the pitch of such signals as speech andthe sounds of various musical instruments by acting upon the periodic or quasi-periodic part of the signal. In the case of speech, thisis part of the signal during which the vocal cords are active. By reducing the similarity of successive cycles of the wave (successive cycles of the fundamental component) to such an extent that a repetition of the wave form will occur only once every n cycles of Athe original fundamental. For example, insofar as processing speech is concerned the one of most interest is one in which n is 2, although it is intended that the present invention also be applicable for other values of n.

An object of this invention, therefore, is to alter the form of the alternate fundamental cycles of the quasiperiodieor periodic part of a signal to be processed. i Another object of the invention is to alter the form of the alternate fundamental cycles of the quasi-periodic or periodic part of a signal, thereby reducing the Vrate at which the wave form repeats itself.

A third object of this invention is to alter the form of alternate fundamental cycles of the quasi-periodic or periodic part of an audio signal to lower the pitch of the audio signal.

A fourth object of the invention is to provide methods and means for controlling signal pitch by separating the signal wave (speech wave, for example) into three signal channels, two to carry the processed speech and a third for the derivation of control signals. In the two signal processing channels, the speech signal is subjected to a different treatment. One method is to leave the speech essentially undistorted in one speech processing channel and to subject the speech to a linear or frequency selective distortion in the other speech processing channel. Another method is simply to adjust two amplifiers, and for each of the speech processing channels, to different gains. 'There are many methods to achieve this objective, the essential aim is to produce two dissimilar wave forms, each of which is intelligible. These two wave forms are delivered to an electronic switch, which alternates 'between them, -rst passing one, then the other. VThe outlowering the subjective periodic signal;v

if- Y 72,94,l$2 lce Patented June 28, 1960 puts of the switch are added together,` and the resulting wave, formed by alternating between the outputs of the two said speech processing channels, is fed to any facility through which the speech is to be transmitted, recorded or reproduced, thereby achieving a method and system for automatically lowering the pitch of a voice or other quasi-periodic signal.

The signals that control the electronic switch, that turn it on and olf at the appropriate times, are derived from the speech wave in the third channel. The signal for the third channel may be the original speech wave, the speech wave in either of the first two channels, or still another modified wave form.' The important thing is for its quasi-periodic parts to have the same periodicity as the signals in the first two channels. The essential requirement is to produce control signals that will turn the electronic switch on and olf at the beginnings of (or at some relatively xed point during) the impulse-cycles of the quasi-periodic wave.

The present invention is intended to applyV to any audio signal which contains periodic or quasi-periodic content.

The features of this invention, which are believed to be new, are set forth with particularity in the appended claims. The invention itself, however, togetherwith further objects and advantages thereof may best be understood by reference to the following description when taken in, conjunction with the accompanying drawings, in which:

Fig. 1 shows in block form the system for automatically pitch of a voice or other quasi- Fig. 2 shows the curves which explain the action of the system shown in Fig; 1; and Y Fig. 3 shows a schematic of the detector circuit utilized in Fig. 1.A

As illustrated in Fig. |1, the speech signal (see Graph l, Fig. 2) is derived from speech signal generator 1 (the speech signal generator may also be a human Voice in conjunction with a microphone or a speech recording) which delivers a series of electrical speech signals which are representative of .thespeech spectrum. The electrical speech signal is fed to isolator 3 by way of line 2. The three output signals of isolator 3 are identical to each other and are shown at Graph 1 of Fig. 2. Y

One of the three output signals is fed by way of line 6 to low-pass iilter 23 and then to detector circuit 9. Lowpass filter 23 is adjusted to eliminate or suppress frequency components above approximately 2,000 or 3,000 cycles per second in order to minimize the effect upon subsequent circuits of the unvoiced or aperiodic signals. Isolator 3 provides a source of low impedance for circuit 9. To explain the operation of detector circuit 9 of Fig. l, we now refer to Fig. 3, capacitor 24 is filled up rapidly when current flows through rectifier 23. The time constant of capacitor 24 and resistor 25 is` very high, however, so that the voltage across capacitor Z4 declines only slowly, as shown in Graph 2 of Fig. 2. The time constant of capacitor 26 and resistor 27, on the other hand, is very low, so that the signal seen at the output of the detector circuit consists of a train of pulses corresponding to the abrupt rises in Graph 2 of Fig. 2. This train of pulses is shown in Graph 3 of Fig. 2.

Now referring to Fig. 1, these pulses are passed through conventional pulse-shaping circuit 10 by way ofline l1 in order to make the pulses uniform in height and abrupt in rise and fall. This is shown in Graph 4 of Fig. 2. The pulses are then passed by way of Y`line 12. to bi-stable multivibrator 13 of the Eccles-Jordan type. This circuit has a single input but two outputs. It separates the pulses into two trains, one consisting of the even members of the incoming train, the other consisting of the odd members of the incoming train. These two trains of pulses are shown in Graphs 5 and 6 of Fig. 2.l Thus, to

summarize the foregoing description, it may be said that the above recited multivibrator action provides a signal for controlling the reciprocal switching of a speech wave between two dissimilar versions thereof. The switching circuitry, per se, is described hereinafter.

ln order to produce two dissimilar signals, two indentical signals from isolator 3 are fed to separate circuits, one by way of line l to distortionless circuit 7 (such as an amplifier), and the other by way of line 5 to time-derivatives circuit The signal in circuit 7 is passed at a predetermined fixed amplitude and the output signal therefrom is identical to the input signal. Time derivative circuit S takes, approximately, the time derivative of the original input signal then utilizes it as the output signal which has about the same output amplitude as distortionless circuit 7. The output of circuit is a signal of the type shown in Graph 7 of Fig. 2, the output for circuit 7 is a signal as shown in Graph l of Fig. 2. The important objective is achieved in that the output signal of circuit S is dissimilar from the original signal and yet has remainedintelligible. The time derivative circuit is being used as an illustrative example. Any pair of distortions that does not destroy intelligibility would be equally suitable. The important thing is to obtain two dissimilar, but still intelligible, versions of the same speech wave.

The two dissimilar signals from outputs of circuits Vand d are fed into switching circuitry which, in the illustrat-ed embodiment of the invention, takes the form of an electronic switch i6 supplied with actuating pulses by way of lines 14 and 15, respectively. Switch lo alternatos between the two dissimilar signals rst passing one and then the other. The operation of switch 16 is contrclled by two trains oi' output pulses from multivibrator i3, one train of pulses, as shown in Graph 5 of Fig. 2, being fed to switch lr6 by way of line 17 and the other as shown in Graph 6 of Fig. 2 beingl fed to switch 16 by way of line l. The pulses on line i7 act as an off-on control whereas the pulses on line 18 serve as ori-off control for switch 16, thereby the output of switch 16 alternates. The operation of switch 16 is controlled by two trains of output pulses from multivibrator 13, one train of pulses as shown in Graph 5 ofFig. 2 being fed to switch le by way of line l?, and the other shown in Graph 6 of Fig. 2 by lway of line i8. The action of switch 16 under control of pulses shown in Graphs 5 and 6 of Fig. 2 wherein the pulses shown in Graph 5 of Fig. 2, are on line l? and pulses shown in Graph 6 of Fig. 2 are on lin-c i3, between the two input signals of switch lo, namely, the signals shown on Graphs l and 7, respectively. The alternating output signals of switch 16 are fed to adder 2l by way of lines 19 and Ztl. These output signals are combined in adder 21 and at its output there is a signal as shown in Graph 8 of Fig. 2. Close examination of this graph shows that during the quasiperiodic interval of the original signal, the processed wave repeats itself only half as often as the original wave. This is due, of course, to the substitution of dissimilar signals for alternate ones of the original set. lf output 3f adder is presented to a device such as earphones, there would be heard a signal whose information content would be substantially identical to the one present at the input source t but differing in that the subjective pitch is lower.

ln the claims wherever the term periodic is used, it is understood to include the audio signal content which repeats itself either periodically or quasi-periodically.

What is claimed is:

l. ln a system for controlling the subjective pitch of a single audio signal containing periodic content having successive cycles of a fundamental frequency component, means for producing three derivative audio signals, each identical with said first-named audio signal, means t0 process two of said three derivative signals so that they are dissimilar but intelligible, one of said dissimilarsignals retaining lits original form and `Othe other of said dissimilar' signals having the form of said successive cycles of said fundamental frequency component altered, means actuated by said third derivative signal to generate control signals representative of the fundamental periods of said electrical signal, and means to combine the odd cycles from one of said two dissimilar signals with the even cycles from the other of said two dissimilar signals, said combining means responsive to said control signals.

2. ln a system for controlling the subjective pitch of a single audio signal as defined in claim 1 wherein said processing means to produce said two dissimilar signals includes amplifying means for one of said two derivative signals, and means to produce a time derivative of the other of said two derivative signals.

3. ln a system for controlling the subjective pitch of a single audio signal as defined in claim l `wherein generating means for said control signals is comprised of detector means for said third derivative signal, pulse-Shaping means adapted to receive said detected signal, multivibrator means actuated by the output pulses from Staid pulse-shaping means, and switching means responsive to the output signals from said multivibration means.

4. in a system for controlling the subjective pitch of a single audio signal containing periodic content, said periodic content having successive cycles of a fundamental frequency component, means to generate an electrical signal representative of said audio signal, means to produce two dissimilar signals which are intelligible versions of said electrical signal, one of said dissimilar signals containingsaid successive cycles of said fundamental component and the other containing a modified form of said successive cycles of said fundamental component, means to generate control signals from said electrical signal, said control signals being representative of each of said successive cycles of said fundamental component, and means to selectively combine alternate cycles of said fundamental component with alternate cycles of said modified form of said fundamental component, said selective combining means responsive to said control signals.

5. In a system for controlling the subjective pitch of a single audio signal containing periodic content having successive cycles of a fundamental frequency component, means to generate an electrical signal representative of said audio signal, said electrical signal being provided with three parallel channels, means to amplify said electrical signal in the first of said channels, means to modify the form of said successive cycles of said fundamental frequency component of said electrical signal in the record of said channels while retaining the intelligibility of said electrical signal, timing signal means in the third of said channels responsive to said electrical signal and generating a control signal for each cycle of said successive cycles of said fundamental frequency component of said electrical signal, and means responsive to said control signals to selectively combine alternate cycles of said successive cycles of said fundamental frequency component of said amplified electrical signal with alternate cycles of said modified form of said successive cycles of said fundamental frequency component of said electrical signal.

6. In a'system for controlling the subjective pitch of a single audio signal as defined in claim 5 wherein the means to modify the form of said successive cycles is comprised of means to generate a time derivative of said electrical signal.

7. ln a system for controlling the subjective pitch of a single audio signal as defined in claim 5 wherein said timing signal means is comprised of means to detect said successive cycles of said fundamental frequency component of said electrical signal, and means to pulse shape said detected signal.

8. ln a system for controlling the subjective pitch of a single audio signal as defined in claim 5 wherein said selective combining means is comprised of an astable i multivibrator responsive to saidcontrol signals, and an electronic switch adopted to receive a pair of signals, one being said amplified electrical signal and the other being said modied signal, said electronic switch being operated by the output signals from said astable multivibrator.

Dudley Nov. 16, 1937- Kendall-et al. Feb. 8, 1938 Dingley Nov. 7, 1944 Corderman June 4, 1946 Gabrilovitch Aug. 16, 1949 Steinberg Apr. 14, 1953 VeauX July 14, 1953 Toulon May 10, 1955 Gabrilovitchtc Mar. 24, 1955 

